1. Field
The present disclosure relates to an ion beam irradiation apparatus, and a mass analyzing electromagnet comprising an analysis tube and usable in the ion beam irradiation apparatus.
2. Description of the Related Art
As an apparatus for performing substrate processing in vacuum by using an ion beam, there has been known an ion beam irradiation apparatus such as an ion implantation apparatus or an ion doping apparatus. Specifically, an apparatus disclosed in JP2536837B has been employed.
The apparatus disclosed in JP2536837B is an ion implantation apparatus in which an ion beam flight tube (also called “analysis tube”) is provided in a sandwiched manner between magnetic poles of a mass analyzing electromagnet. JP2536837B points out as a problem the following phenomenon occurring within the analysis tube.
Unnecessary ion components and neutral particles separated from an ion beam through mass analysis in the mass analyzing electromagnet adhere to an inner wall surface of the analysis tube in a first region thereof intersecting with a direction of mass-based separation (mass-based separation direction) of the ion beam. Such an adhered substance to the inner wall surface will be accumulated over time to form a deposit. Then, during operation of the apparatus, unnecessary ion components and neutral particles in an ion beam can collide with the deposit, thereby causing peeling of the deposit from the inner wall surface of the analysis tube. In this case, the peeled deposit is ejected toward a target (a substrate such as a silicon wafer or a glass substrate) and mixed in the target, resulting in contamination of the target.
As a measure against this problem, JP2536837B proposes configuring the analysis tube such that the first region of the inner wall surface thereof intersecting with the mass-based separation direction in the mass analyzing electromagnet is kept from facing a beam path downstream of the analysis tube, for example, by forming the first region of the inner wall surface into a wedge shape, as illustrated in FIGS. 10 and 14 thereof.
Generally, in an ion beam transport path between an ion source and a processing chamber, the mass analyzing electromagnet occupies a relatively large installation area, as compared to other optical elements. This is because the mass analyzing electromagnet is required to cause an ion beam to be transported with a large gyration radius over a sufficient distance in order to remove unnecessary ion components therefrom.
Meanwhile, due to the space-charge effect, an ion beam diverges to a greater or lesser extent depending on its energy. The influence of spreading of an ion beam caused by the space-charge effect appears more significantly along with an increase in ion beam transport distance. In the analysis tube, the ion beam transport distance is greater than in other optical elements, and therefore the influence of spreading of an ion beam caused by the space-charge effect becomes more significant.
As means to permit divergence of an ion beam caused by the space-charge effect, it is conceivable to enlarge a spatial zone inside the analysis tube. In this case, however, what is necessary is not only to simply enlarge the spatial zone inside the analysis tube. For example, the following problems are assumed. If the spatial zone inside the analysis tube is excessively enlarged, a distance between the magnetic poles disposed outside the analysis tube is increased, and thereby a magnetic field distribution within the analysis tube becomes non-uniform. The non-uniform magnetic field distribution is likely to cause a situation where a shape of an ion beam is changed during deflection of the ion beam, causing a negative influence on mass analysis on the ion beam. Further, the inside of the analysis tube serving as the ion beam transport path needs to be maintained in a vacuum state. Thus, the excessive enlargement of the spatial zone inside the analysis tube gives rise to a need to equip a vacuum pump having a high evacuation capability in order to keep a vacuum pressure constant. The excessive enlargement of the spatial zone inside the analysis tube further causes an increase in time period after opening the ion implantation apparatus to atmospheric air once to perform maintenance of an inside of the apparatus through until an internal atmosphere of the apparatus is returned to an original vacuum pressure.
In view of the above problems, the spatial zone inside the analysis tube of the mass analyzing electromagnet is designed to have a size slightly greater than a design size of an ion beam to be subjected to passing therethrough.
Considering an overall spreading of an ion beam passing through the analysis tube, in the mass-based separation direction, spreading of the ion beam caused by separation of unnecessary ion components and neutral particles from the ion beam is significantly greater than spreading of the ion beam caused by the space-charge effect. With this in mind, in JP2536837B and many other related art techniques, a measure has been taken based on an idea of how to prevent a deposit caused by unnecessary ion components and neutral particles separated from an ion beam, i.e., a deposit occurring in the first region intersecting with the mass-based separation direction, from being ejected toward the downstream side.
However, an ion beam subjected to the influence of the space-charge effect is not spread along one direction but spread all around. Thus, spreading of an ion beam in any direction other than the mass-based separation direction along which unnecessary ion components and neutral particles are separated from the ion beam is largely influenced by divergence arising from the space-charge effect, unless the ion beam is intentionally subjected to divergence.
The size of the spatial zone inside the analysis tube is only slightly greater than that of an ion beam, as mentioned above. Thus, a peripheral end of an ion beam spread by the space-charge effect collides with the inner wall surface of the analysis tube in a second region thereof intersecting with any direction other than the mass-based separation direction, and thereby chemical components contained in the ion beam adhere to the second region of the inner wall surface. This leads to a concern that such chemical components will be accumulated over time, and the resulting deposit can be peeled from the inner wall surface due to a collision with a peripheral end of an ion beam at a certain timing, etc., and ejected toward a target, resulting in contamination of the target. For example, in the case where ion implantation is performed using different types of ions, if previously used ions are peeled from the inner wall surface of the analysis tube during ion implantation subsequently performed using different ions, and mixed in a target, the target is undesirably contaminated. From this point of view, it is also necessary to consider a measure effective in the second region intersecting with any direction other than the mass-based separation direction to fully prevent contamination of a target.
Moreover, it is difficult to exactly reproduce an ideal design shape of an ion beam by controlling respective applied voltage to electrodes constituting an extraction electrode system of an ion source. Therefore, an ion beam extracted from the extraction electrode system slightly diverges. Thus, due to an influence of such a diverged component, when an ion beam passes through the analysis tube of the mass analyzing electromagnet, a peripheral end of the ion beam undesirably collides with the inner wall surface of the analysis tube. As a result, the same problem as that in the aforementioned divergence caused by the space-charge effect occurs.